Electron beam heating apparatus



@mawn mil May 27, 1969 c. w. HANKS 3,446,934

ELECTRON BEAM HEATING APPARATUS Filed Jan. 50, 1968 Sheet of 2 d-c power supply FIG. I.

power pp y INVENTOR.

CHARLES W. HANKS Bx I 1 \L M ATTORNEYS y 1969 c. w. HANKS 3,446,934

ELECTRON BEAM HEATING APPARATUS Filed Jan. 30, 1968 Sheet g of 2 FIG. 3.

FIG. 4.

!. \"ENTOR.

CHARLES W. HANKS 924.14. ATTORNEYS United States Patent 3,446,934 ELECTRON BEAM HEATING APPARATUS Charles W. Hanks, Orinda, Califi, assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Jan. 30, 1968, Ser. No. 701,727 Int. Cl. B23k 9/00; H0510 1/00 U.S. Cl. 219-121 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electron beam heating apparatus and, more particularly, to apparatus for heating a'target surface by means of an electron beam which is swept over the target surface.

High vacuum electron beam furnaces have been used for some time in the vacuum processing of various materials. Such :furnaces are utilized, for example, to produce vapors of metals and other materials for deposition on a substrate. Such furnaces are also used in the melting and casting of metallic ores to obtain relatively pure metals or alloys and in the melting of materials other than metals, such as ceramics and plastics.

Electron beam furnaces utilize electron gun assemblies including one or more electron beam guns for producing high energy electron beams. The electron gun assemblies also include means for directing such beams to a target to be heated. Transverse magnetic fields may be used for deflecting the beam through a curving path permitting the electron gun to be placed in a location wherein it is less likely to be damaged by direct condensation of evaporant or by ion bombardment. Focusing action is achieved in the direction transverse to the plane of the curving electron beam path (lateral focusing) when the lines of flux in the transverse magnetic field are bowed to produce a barrel-shaped magnetic field.

Under certain operating conditions, it is desirable to repeatedly sweep the electron beam back and forth across the surface of the target to be heated in order to maintain a generally uniform temperature across the entire surface. One form of apparatus for successfully accomplishing such repeated sweeping, referred to as dithering, is shown and described in U.S. Patent No. 3,235,647. An electrical circuit for supplying power to produce the desired electromagnetic field for causing dithering is shown and described in copending U.S. patent application No. 480,287. Apparatus constructed in accordance with the foregoing invention operates to produce sweeping or dithering of the beam by varying the direction of the lines of force of the deflecting transverse magnetic field with respect to the path of the beam (skewness).

Although the foregoing described appartus has been sucessful in many applications, some limitations may be encountered Where the electron beam path is relatively short. Such short beam path lengths may exist in relatively small size electron beam apparatus wherein the beam is produced in an electron beam gun adjacent to and slightly below the surface of a crucible, and is deflected through approximately 180 by means of a transverse field across the top of the crucible to impinge upon the surface of material contained in the crucible. Under 3,446,934 Patented May 27, 1969 certain circumstances, selective variation of the skewness of the deflecting magnetic field across the top of the crucible may not produce sufficient sweep to cover the entire surface of the target.

Accordingly, it is an object of this invention to provide improved electron beam apparatus for heating a target surface.

Another object of the invention is to provide improved electron beam heating apparatus having provision for sweeping or dithering the electron beam on the target surface.

Another object of the invention is to provide electron beam apparatus capable of producing a relatively wide sweep within a relatively short electron beam path length.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

FIGURE 1 is a plan view of one embodiment of electron beam heating apparatus construced in accordance with the invention;

FIGURE 2 is a sectional view taken along the line 2-2 of FIGURE 1;

FIGURE 3 is a schematic plan view of a portion of FIGURE 1 illustrating a transverse magnetic field established by the apparatus of the invention under a particular condition of operation;

FIGURE 4 is a schematic plan view similar to FIG- URE 3 illustrating two transverse magnetic fields established by the apparatus of the invention in a second condition of operation; and

FIGURE 5 is a schematic plan view of another portion of FIGURE 1 illustrating the path of sweep of the area of electron beam impingement on a target surface.

Very generally, the apparatus of the invention comprises an electron beam gun 11 for producing a beam 12 of electrons directed in an initial path. A pair of main pole pieces 13 and 14 are positioned on opposite sides of a target surface 16. First coil means 17 and 18 are provided for establishing a first transverse magnetic field extending between the main pole pieces in the path of the electron beam to thereby deflect the beam 12 of electrons onto the target surface 16. A third pole piece 19 is positioned on the opposite side of the initial beam path with respect to the target surface. A second coil means 21 is provided Which establishes a second magnetic field extending between the third pole piece and one of the main pole pieces in the path of the electron beam. By varying the strength and direction of the sec ond field, the beam may be moved laterally over the target surface.

Referring now more particularly to FIGURES 1 and 2, molten material 22, the surface 16 of which is heated by the electron beam 12, is contained within a crucible 23. The crucible is supported on a frame 24 which rests upon a base plate 26. The crucible is disposed within a high vacuum environment and is provided with coolant passages 27 for cooling the crucible during melting operations. Accordingly, a skull 28 of solid material forms between the molten material 22 and the interior 'wall of the crucible 23 to prevent interaction between the crucible and the molten material.

The electron beam 12 for heating the surface 16 of the molten material 22 in the crucible 23 is produced by the electron beam gun 1 1. The gun 11 includes an elongated emitter 28 comprised of tungsten or a material having similar properties and, when a heating current is passed therethrough, emits electrons. The emitter 28 is supported, by suitable means not illustrated, in an elongated recess 29 formed in a backing electrode 30. The unillustrated emitter supports may also provide electrical contact for passing the heating current through the emitter. The backing electrode 30 is mounted on a support frame 31 which, in turn, is carried by the base plate 26.

The electrons in the beam are accelerated by an accelerating electrode or anode 32. The anode 32 consists of a metal plate having an opening 33 therein through which the beam passes. One edge 34 of the metal plate 32 is turned downward and is bolted to a vertical support plate 35. The support plate has two inwardly extending legs 36 which are bolted to the base plate 26.

The backing electrode 30 and the emitter 28 are maintained at a negative potential with respect to the accelerating electrode 32, and the recess 29 is shaped such that the electrons emitted by the emitter are directed out of the open end of the recess in the shape of a beam. Because of the elongated shape of the emitter 28 and the elongated shape of the recess 29, the beam is in the form of a ribbon (i.e., having a narrow rectangular cross section) as it leaves the anode opening. The initial path of the beam, as it leaves the anode opening, is substantially perpendicular to the plate 32.

After leaving the anode opening 33, the beam 12 is deflected to pass through a curving path and make a change in direction of slightly less than 180 by a first magnetic field established between the vertically extending generally truncated pyramid shaped spaced main pole pieces 13 and 14. The pole pieces 13 and 14 are disposed above the target surface and are mounted on a U-shaped yoke 37 of a material having low magnetic reluctance. The yoke 37 includes a vertical leg or core 38 which extends upwardly and connects at its cylindrical upper end with the pole piece 13. A further vertical leg or core 39 extends upwardly and connects at its cylindrical upper end with the pole piece 14. A horizontal rectangular leg or bar 41 having the same width as the vertical legs connects the lower end of the two vertical legs 38 and 39 of the yoke 37. The electromagnetic coil 17 surrounds the vertical leg 38, and the electromagnetic coil 18 surrounds the vertical leg 39. A further horizontal rectangular leg or bar 42 extends perpendicularly of the horizontal leg 41, intersecting the latter at approximately the center thereof. The horizontal leg 42 passes beneath the electron beam gun 11 and through an opening 44 in the vertical support plate 35. The yoke 37 and the horizontal leg 42 are supported by a suitable frame 45 of a material having a high magnetic reluctance which, in turn, is carried by the base plate 26.

A vertical cylindrical leg 46 extends upwardly from the far end of the horizontal leg 42, and the pole piece 19, which is in the form of a bar, extends horizontally from and is connected to the top of the vertical leg 46. The level of the pole piece 19 is above the level of the accelerating anode plate 32 and terminates near the edge of the path of electrons issuing from the anode opening 33. The second coil means comprises a coil 21 mounted surrounding the vertical leg 46.

Suitable D-C energizing currets are supplied to the electromagnetic coils 17 and 18 by D-C power supplies 47 and 48 to thereby establish the magnetic field between the main pole pieces 13 and 14, as indicated by the lines of force 51 in FIGURE 3. Because of the pyramid shape of the pole pieces 13 and 14, the lines of force 51, as may be observed in FIGURE 3, are bowed in the region directly above the emitter 28. In this region, the bowing is both toward the third pole piece 19 and upwardly out of the plane of the paper. Above the crucible 23, however, the lines of force 51 are bowed away from the third pole piece 19 and upwardly out of the plane of the paper. Accordingly, the field established by the pole pieces 13 and 14 approximates a barrel shape and therefore produces laeral focusing of the beam both as it is rising from the anode opening 29 and as it drops back to the surface 16 of the melt 22. This may be observed in FIGURE 3 from the dotted lines 12, representing the edges of the ribbon-shaped electron beam. The beam converges to an impact pattern in the shape of a spot 53 on the surface 16 of the molten pool. The line where the pyramid faces intersect on each pole piece is positioned adjacent the region between the electron beam gun and the target surface. The highest point of the barrel shaped field is therefore above this region, as is the highest point in the arch of the beam, as may be seen in FIGURE 2. A similar barrel shaped field may be poduced by conical pole pieces, if desired.

By referring to FIGURE 4, the influence of energization of the third pole piece 19 may be observed. With the third pole energized north, by suitable current direction in the coil 21, as supplied by a supply 49, a second magnetic field is established extending between the north pole piece 19 and the south pole piece 14. The main magnetic field is represented in FIGURE 4 by the lines of force 52. The second magnetic field is represented by the lines of force 54. It may be seen that the two magnetic fields merge into each other and influence each other in direction of lines of force. The beam 12 undergoes lateral deflection, as illustrated, due to the direction of the lines of force of the fields 52 and 54. The field 54 exerts the predominant influence on the lateral deflection of the beam, both through the direction of its own lines of flux and through its Warping of and merging with the lines of flux of the field 52.

With the polarity of the pole piece 19 reversed, the second magnetic field extends between the pole piece 13 and the pole piece 19. The beam is, accordingly, deflected toward the opposite edge of the pool surface 16. If desired, the beam may be dithered on the pool surface by providing an alternating current from the source 49. By providing an alternating current which is constantly varying in magnitude from the source 49, the amount of deflection imparted to the beam by the field 54, dependent upon the strength of that field, may be varied to sweep the beam laterally across the surface. A typical sweep pattern may be observed in FIGURE 5, the two beam spots 53a and 53b representing the extremes of sweep. The beam sweep pattern may be moved more to the center of the crucible surface 16 (to the right in FIGURE 5) by increasing the current in the coils 17 and 18 and hence the strength of the deflecting field between the pole pieces 13 and 14. The stronger the field, the greater the total amount of deflection through the arcuate beam path. The arcuate nature of the sweep pattern results because the second varying deflecting field varies both in direction and strength, the latter factor affecting the arcuateness of the beam path.

Accordingly, several factors are involved in controlling the beam deflection in the foregoing described apparatus. The transverse nature of the fields 52 and 54 cause an approximate deflection of the beam 12 in a plane perpendicular to the paper and the emitter 28' in FIG- URE 4. This deflection may be varied by changing the D-C current in the coils 38 and 39, and undergoes a variation to follow the curvature of the crucible because of the changing strength of the field 54. Lateral focusing or convergence of the beam perpendicular to the said plane occurs due to the barrel shape of the field 52. Deflection of the beam in the plane of the paper in FIGURE 3 to cause sweeping or dithering is produced by the changing direction of the field 54, due to the direct influence of the field 54 on the beam, and due to the Warping of the field 52 caused by the field 54.

The second transverse field established by the pole piece 19 may have lines of flux which are almost parallel with the beam, i.e., extending from the pole piece 19 to near the top of one of the pole pieces 13 and 14. Because of the pyramidal shape of the pole pieces, however, and because it is usually desirable to locate the pole piece 19 as low as possible to keep it clear of condensation, such parallel lines of force are relatively weak because of the greater gap and therefore have a negligible effect on the beam.

It may therefore be seen that the invention provides an improved electron beam apparatus for heating a target surface. The invention provides for close control over the electron beam by a relatively simple structure. The result is highly satisfactory in the control of dithering of the electron beam at exceptionally low cost. Although the invention has been described in connection with repetitive beam sweeping or dithering, the power supply 49 may be constructed to provide a regulated D-C of either polarity for regulating beam impact to a precise position on the surface 16.

Various modifications of the invention will be apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. Apparatus for heating a target surface, comprising, an electron beam gun for producing a beam of electrons directed in an initial path, a pair of main pole pieces positioned on opposite sides of the target surface, first coil means for establishing a first transverse magnetic field extending between said main pole pieces in the path of the electron beam, a third pole piece positioned on the opposite side of the initial beam path from the target surface, and second coil means for establishing a second transverse magnetic field extending between said third pole piece and one of said main pole pieces and in the path of the electron beam.

2. Apparatus according to claim -1 wherein said second coil means are adapted to be energized selectively in opposite directions to thereby select the one of said main pole pieces to which the second transverse field extends.

3. Apparatus according to claim 1 including means for providing an alternating current passing through said 1 second coil means.

4. Apparatus according to claim 1 including a yoke providing a first low reluctance path extending between said main pole pieces, and second and third low reluctance paths extending between said third pole piece and each of said main pole pieces.

5. Apparatus according to claim 4 wherein said yoke includes three intersecting legs terminating at said pole pieces, respectively, wherein said first coil means comprise a pair of coils, one surrounding each leg associated with said main pole pieces, and wherein said second coil means comprise a coil surrounding the leg associated with said third pole piece.

6. Apparatus according to claim 1 wherein said pole pieces are positioned such that the second transverse magnetic field intersects the path of the electron beam prior to the first transverse magnetic field.

7. Apparatus according to claim 1 wherein said main pole pieces are shaped to produce a barrel shaped first transverse magentic field disposed with its highest point positioned above the region between said electron beam gun and the target surface.

8. Apparatus according to claim 7 wherein said main pole pieces are pyramid shaped, each having surfaces intersecting at a line adjacent the region between said electron beam gun and the target surface.

References Cited UNITED STATES PATENTS 3,068,309 12/1962 Hanks 219-121 3,132,198 5/1964 Du Bois et al 219l21 3,235,647 2/1966 Hanks 219121 3,303,320 2/1967 Muller 219l21 3,390,222 6/1968 Anderson 219-421 3,390,249 6/1968 Hanks 219121 RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner.

US. Cl. X.R. 13-31 

